1. FIELD OF THE INVENTION
The present invention relates to a ceramic circuit substrate having on its surface an external resistor covered with a glass overcoat. More particularly, the present invention is concerned with a ceramic circuit substrate having an external resistor which stably maintains an accurate resistance value obtained by trimming.
2. DESCRIPTION OF PRIOR ART
Besides the internal resistor disposed between the layers of a multilayer circuit substrate, a ceramic circuit substrate for use in integral circuits is provided with a circuit comprising an external resistor and a conductor pattern printed on the surface of the ceramic circuit substrate, which contributes toward imparting an advanced function to the ceramic circuit substrate and reducing the production cost.
In the formation of a resistor on a substrate surface, generally, a conductive substance such as RuO.sub.2 is added to a glass composition, rendered pasty, printed and sintered into the desired resistor. In the formation of the resistor, occasionally, printing is effected so as to cover the resistor with a glass material and followed by firing to form an overcoat in order to protect the resistor and to improve the weather resistance thereof. The obtained resistor has its resistance value finely adjusted by laser trimming.
Generally, an external resistor used in a ceramic circuit substrate is formed by firing a resistor at 800 to 900.degree. C., printing a low-melting-point glass overcoat thereon, and firing at 500 to 600.degree. C. The coefficient of thermal expansion of RuO.sub.2 used in the resistor is about 6.0.times.10.sup.-6 /.degree. C. and that of the resistor comprising the same is 5.5.times.10.sup.-6 /.degree. C. to 7.0.times.10.sup.-6 /.degree. C. In contrast to this, the coefficient of thermal expansion of an alumina substrate is as large as 7.0.times.10.sup.-6 /.degree. C. to 8.0.times.10.sup.-6 /.degree. C., so that a compressive force is applied to the resistor formed on the alumina substrate. As a result, laser trimming of the resistor does not promote cracking.
In accordance with the miniaturization of electronic appliances and the high-density packaging therein, there is the tendency that the ceramic substrate is also provided in multilayer form to comply with high-density packaging and that use is made of substrate materials which each have a low coefficient of thermal expansion for mounting silicon chips thereon. As such circuit substrates, low-temperature firable substrates are used. However, when the resistor is formed on a substrate having a coefficient of thermal expansion smaller than that of the resistor, a tensile force is applied to the resistor after the firing, thereby causing the resistor to crack during laser trimming or thereafter.
The above situation will be described with reference to the drawings. FIG. 1 is a plan of one form of a conventional external resistor disposed on a ceramic circuit substrate of a ceramic circuit substrate, and FIG. 2 is a sectional view thereof. A wiring material such as a metal paste is printed on a ceramic substrate surface 1, thereby forming a conductor pattern 2 on the surface. Part thereof constitutes an electrode for a resistor 3. The resistor 3 is composed of glass components having a conductive material such as a metal added thereto. The upper part thereof is covered with an overcoat 4 composed of glass materials. The resistor 3 and the overcoat 4 constitute an external resistor 71. The overcoat 4 may either cover each individual resistor 3 a little wider than the same or uniformly cover a wide area of not only a plurality of resistors 3 but also a conductor pattern 2. When the overcoat covers such a wide area, via holes can be provided at suitable positions to thereby attain the continuity with the outside.
Laser trimming of such an external resistor 7 leads to formation of a trimming channel 5 in the overcoat 4 and the resistor 3, as shown in the figures.
Although laser trimming is generally conducted while measuring the resistance value exhibited by the resistor, the presence of expansion force in the resistor not only interferes with such precision trimming but also generates microcracks 6 from the trimming channels. Also, even if there is no occurrence of cracks during trimming, cracks may occur during the use as a product.